You probably have to create the custom_components folder, since it generally doesn’t exist by default. It needs to be in the same directory where you configuration.yaml file exists.
If you’re not comfortable with doing everything necessary to get custom components installed, you can just use standard HA components like input_number, input_text, etc. That’s what many, if not most, people do (that aren’t using node-red, appdaemon, etc.)
Basically yes. Depending on details you may have to deal with ownership, permissions, etc. Also don’t forget to add to your config per the install instructions.